Coalesce concurrent connectToAgent() calls in SessionManager

[stefanpieter]

Summary

SessionManager.connectToAgent(agentName) should coalesce concurrent calls for the same agent behind a shared pending promise instead of allowing duplicate in-flight connects.

Why

When multiple startup/view-hydration paths trigger connectToAgent() at nearly the same time, the extension can race into duplicate spawn/connect/session creation work. This creates flakiness and slows local iteration/debugging.

Proposal

• track in-flight connectToAgent(agentName) operations in a per-agent map
• if a second call arrives for the same agent while the first is still pending, return the same promise
• clear the pending entry on resolve/reject

Acceptance criteria

• only one underlying connect/spawn/session-new path runs for concurrent same-agent calls
• parallel callers all receive the same eventual result
• rejection cleanup works correctly
• regression test proves only one connection flow occurs under Promise.all([connectToAgent("Hermes Agent"), connectToAgent("Hermes Agent")])

Notes

This is both a correctness fix and a developer-velocity improvement because duplicate connects make the VS Code session state harder to reason about during local iteration.